An organic substance capable of transporting electronic charges by positive hole or electron can be used as an organic semiconductor, and can be used as a material for organic electronic devices, such as photoreceptors for copying machines, photosensors, organic EL (electroluminescence) devices, organic transistors, organic solar cells and organic memory devices.
Such a material may generally be used in the form of an amorphous thin film or a polycrystalline thin film. On the other hand, in recent years, there has been found an electronic conduction exhibiting a much higher mobility than that of an amorphous organic semiconductor occurring in a liquid crystal phase of a liquid crystal substance, which has heretofore been considered as an ion conductive substance, and further it has been recognized that the liquid crystal phase is usable as an organic semiconductor.
Such a liquid crystal substance may be positioned as a new type of organic semiconductor, which is capable of forming a molecular condensed phase (i.e., liquid crystal phase), which is oriented in a self-organizing manner and exhibits a high mobility (10−4 cm2/Vs to 1 cm2/Vs). In addition, such a liquid crystal substance has been found to have an excellent property, which conventional amorphous organic semiconductor materials or crystalline organic semiconductor materials cannot exhibit. More specifically, the liquid crystal substance is characterized in that an orientation defect specific to a liquid crystal, such as domain interface or disclination, scarcely allows for the formation of an electrically active level. In practice, an electronic device such as photosensor, electrophotographic photoreceptor, organic EL device, organic transistor and organic solar cell is being produced on trial by using a liquid crystal phase as an organic semiconductor.
The liquid crystal substance has a prominent characteristic such that in general, molecular orientation, which is hard to be controlled in a non-liquid crystal substance, can easily be controlled in a liquid crystal phase. For example, a rod-like liquid crystal substance generally has a tendency that, when the liquid crystal substance is injected between two substrates as in the case of a liquid crystal cell, the liquid crystal molecules are liable to be oriented in a state wherein the molecular major axis thereof lies almost parallel to the substrate surface at a liquid crystal phase temperature, and when the liquid crystal substance is applied onto a substrate, the molecules are liable to be oriented in a state wherein the molecular long axis thereof rises almost perpendicular to the substrate surface. The utilization of this property makes it easy to produce a thin film (crystalline thin film) having a controlled molecular orientation not only in a liquid crystal phase but also in a crystal phase by lowering the temperature of the thin film liquid crystal which has been oriented at a liquid crystal phase temperature, to thereby cause a phase transition to a crystal phase. This is difficult to be realized in the case of an ordinary non-liquid crystalline organic material.
It has been reported that a crystal thin film excellent in the crystallinity or flatness can be produced, when a liquid crystal thin film (i.e., a thin film in the state of a liquid crystal phase) of a liquid crystal substance is utilized as a precursor at the formation of a crystal thin film by using the above-described characteristics.
According to this technique, a uniform film excellent in the surface flatness may be obtained by forming a liquid crystal film at a liquid crystal phase temperature, and then cooling the resulting liquid crystal film to a crystallization temperature. In view of such an applicability of the liquid crystal substance to an electronic device as an organic semiconductor material, not only in the form of a liquid crystal thin film but also in the form of a crystalline thin film, the liquid crystal substance may be a material having a high degree of freedom as an organic semiconductor (see, for example, Non-Patent Document 1: Advanced Materials, electronic edition, 25 FEB 2011, DOI: 10.1002/adma.201004474).
However, when a liquid crystal substance is intended to be used as an organic semiconductor, it is necessary to obtain a liquid crystal substance exhibiting a high electron mobility. In this connection, in an attempt to obtain a substance exhibiting a high electron mobility, there is posed a problem such that what kind of a substance should be synthesized.
Heretofore, various materials have been synthesized as a liquid crystal substance, but the target thereof has been substantially limited to a nematic liquid crystal to be used as a display material for a display device utilizing optical anisotropy. Accordingly, a guideline for molecular design of a liquid crystal substance, which is suitable for the liquid crystal substance as an organic semiconductor, that is, a way of thinking in which the liquid crystal substance may be synthesized, has never been clarified.
Accordingly, in the prior art, in an attempt to synthesize a novel liquid crystal substance exhibiting a high electron mobility, there has been no method except for a trial-and-error method wherein, basically, an aromatic ring-containing core structure and a hydrocarbon chain is combined so as to select a desired chemical structure, and after the actual synthesis of the substance, the liquid crystal phase exhibited by the substance is examined. In addition, a useful guideline has not been provided for designing a structure suitable for an organic semiconductor exhibiting a high mobility, and the synthesis has encountered great difficulty in developing such a material.